My favorite response to that (I forget where I saw it first) was always, "Well John is a good friend of mine and I trust his judgement. If he was lucid and jumped off a bridge, there must be a damn good reason so yes, I'd probably jump off too."

To be fair odds are your friend is going to make some form of statement regarding jumping off the bridge which'll make it pretty easy to identify if you should do the same. "Fuck the train's coming" and "Goodbye" are two very different scenarios that will let me quickly decide if jumping off the bridge is a wise move.

Sorry, I more meant that your friend isn't likely to jump off a bridge without saying something to you first, and that's going to make it a lot easier to make conclusions regarding their decision to do so.